1. Field of the Invention
This invention relates in general to apparatus which may be disconnected and reconnected in a wellbore by means of uni-directional angular manipulations of drill pipe without removing the drill pipe from the well. In particular, the invention relates to apparatus for testing blowout preventors in a subsea blowout preventer stack. Still more particularly, the invention relates to a tool facilitating testing of a shear ram blowout preventer in a subsea blowout preventer stack.
2. Description of the Prior Art
Blowout preventer stacks are used on the sea floor for controlling a well during floating drilling rig operations. The blowout preventer stack is attached to a wellhead on the ocean floor from which well casing is hung and cemented into the well bore. Attached to the top of the blowout preventer stack is a riser system extending to a floating drilling vessel such as a semi-submersible drilling platform or a drilling ship.
The individual blowout preventers are in general required to be tested by regulatory authorities in the interest of safety and ecology. Such tests have been conducted in the past by lowering a test tool from the drilling rig through the riser and through the open bores of the individual blowout preventers in the stack for sealing in the wellhead below the blowout preventer stack. The individual blowout preventers, with the exception of the shear ram, have been tested by pressuring the stack through the means of a choke or kill line with pressurized drilling fluid. Each individual blowout preventer is tested in turn by closing the preventer about the drill pipe and determining whether or not the preventer maintains the pressure from below. In the past the shear ram preventer in the stack has simply not been tested (because its shearing blades would shear the drill pipe) or has been tested to a low pressure against a cement plug in the casing while drill pipe was removed from the well bore.
With increasing interest in countries demanding the utmost in safety of its offshore waters, some governments have begun demanding that all elements in the blowout preventer stack be tested periodicly to full rated working pressure. Such a requirement has demanded that the shear ram blowout preventer also be tested.
A prior method and apparatus for testing the blowout preventers, including the shear ram blowout preventer, has been used. Such an apparatus has included a sealing test tool which is lowered by means of a drill pipe through the aligned bores of the individual blowout preventers of the subsea blowout preventer stack until it is landed in the wellhead. Such a sealing test tool has included a bore therein for communication with the interior of the drill pipe. The bore had been prepared for insertion of a check valve adapted to prevent downward fluid flow yet allowing flow to the interior of the drill pipe from beneath the sealing test tool in the wellhead. Such a check valve has enabled operators to check the efficiency of the sealing tool in sealing about the wellhead. When the stack is pressured by means of a choke or kill line, leakage below the sealing test tool could be detected in the interior of the drill pipe at the surface because of flow upwardly through the check valve.
Also included in the apparatus has been a backout sub connected between the sealing test tool and the drill pipe which may be disconnected, leaving the sealing test tool in the wellhead yet allowing the drill pipe to be raised above the shear ram blowout preventer for its testing. Such a backout sub has been provided with left hand threads connecting an upper part of the sub with a lower part such that the drill pipe may be disconnected from the lower part of the backout sub and the attached sealing test tool in the wellhead by turning the drill pipe to the right, thereby disconnecting the drill pipe and the upper part of the sub from the lower part of the sub. In order to reconnect the upper part of the sub and the drill pipe with the lower part of the sub, it has in the past been necessary to "trip" the drill pipe. That is, the drill pipe is raised joint by joint to the surface, such that a connector with right hand threads may be provided on the upper part of the sub. When the drill pipe is lowered (again, joint by joint) back down through the riser and through the blowout preventer stack, it was then possible to reconnect the upper part of the sub to a second set of right hand threads on the lower part of the sub. A right hand turning of the drill pipe could again connect the upper part of the sub with the lower part of the sub.
The industry has recognized the advantage of being able to disconnect from the lower part of the sub and reconnect to the lower part of the sub after the shear ram blowout preventer has been tested. A single set of left hand threads could be provided to connect the upper and lower parts of the sub. Such a connection would require disconnection by turning to the right and reconnection by turning to the left, a procedure which would eliminate the necessity of tripping the drill pipe. Turning only to the right to disconnect and then reconnect the upper part of the sub and the lower part of the sub obviates the possibility of disconnecting one of the joints of the drill pipe which are typically connected with right hand threads. As described above, apparatus used for testing a shear ram blowout preventer in a subsea blowout preventer stack requiring only right hand turning for disconnection and reconnection has required the tripping of the driill pipe. In deep water, tripping of the drill pipe may cause considerable delay in the testing process, a process which must be accomplished periodicly during drilling. Drilling delays in offshore operations are very expensive.
Prior apparatus for testing subsea blowout preventer stacks has included a sliding sleeve ported sub connected in the drill pipe string above the backout sub to allow the tool string bore to drain during retrieval after testing so that the pipe is not raised to the surface with drilling fluid trapped in its interior.
It is thus an object of the invention to provide a tool having the means to disconnect and reconnect a sealing test tool in a wellhead below a subsea blowout preventer stack by turning a drill pipe connected to the upper part of the tool in a single direction without tripping the drill pipe.
It is a further object of the invention to provide a tool useful in testing a shear ram blowout preventer in a subsea blowout preventer stack in which drill pipe is turned to the right to disconnect a top portion of the tool which is then raised above the blind ram to be tested and then lowered for reconnection with the lower portion of the tool by again turning the drill pipe to the right without the necessity of tripping the drill pipe to the surface of the drilling rig.
It is another object of the invention to provide drain ports in the tool such that when the tool is retrieved to the surface, the drain ports are open to the exterior of the tool thereby assuring that drilling fluid is not trapped in the interior of the drill pipe as it is being raised to the surface.
It is a further object of the invention to provide a bore in the tool in which a check valve may be inserted allowing upward flow through the bore of the tool and the drill pipe if pressurized fluid were to leak past the sealing test tool so that it can be detected at the surface.